STRONG CONVERGENCE BOUNDS OF THE HILL-TYPE ESTIMATOR UNDER SECOND-ORDER REGULARLY VARYING CONDITIONS ZUOXIANG PENG AND SARALEES NADARAJAH Received 22 April 2005; Revised 7 July 2005; Accepted 10 July 2005 Bounds on strong convergences of the Hill-type estimator are established under second- order regularly varying conditions Copyright © 2006 Z. Peng and S. Nadarajah. This is an open access article distributed un- der the Creative Commons Attribution License, which permits unrestricted use, distribu- tion, and reproduction in any medium, provided the original work is properly cited. 1. Introduction Suppose X 1 ,X 2 , are independent and identically distributed (iid) random variables with common distribution function (df) F.LetM n = max{X 1 , ,X n } denote the maxi- mum of the first n random variables and let w(F) = sup{x : F(x) < 1} denote the upper end point of F. The extreme value theory seeks norming constants a n > 0, b n ∈and a nondegenerate df G such that the df of a normalized version of M n converges to G,that is, Pr  M n −b n a n ≤ x  = F n  a n x + b n  −→ G(x) (1.1) as n →∞. If this holds for suitable choices of a n and b n then it is said that G is an extreme value df and F is in the domain of attraction of G,writtenasF ∈ D(G). For suitable constants a>0andb ∈,onecanwrite G(ax + b) = G γ (x) =exp  − (1 + γx) −1/γ  (1.2) for all 1 + γx > 0andγ ∈.Forγ>0, (1.1)isequivalentto lim t→∞ U(tx) U(t) = x γ , (1.3) where U(t) = (1/(1 −F)) ← (t) = inf{t ∈ R :1/(1 −F(x)) ≥ t}, that is, U(t)isaregularly varying function at infinity with index γ. Hindawi Publishing Corporation Journal of Inequalities and Applications Volume 2006, Article ID 95124, Pages 1–7 DOI 10.1155/JIA/2006/95124 2 Strong convergence bounds of the Hill-type estimator The distribution given by (1.2) is known as the extreme value distribution. Its practi- cal applications have been wide-ranging: fire protection and insurance problems, model for the extremely high temperatures, prediction of the high return levels of wind speeds relevant for the design of civil engineering structures, model for the extreme occurrences in Germany’s stock index, prediction of the behavior of solar proton peak fluxes, model for the failure strengths of load-sharing systems and window glasses, model for the mag- nitude of future earthquakes, analysis of the corrosion failures of lead-sheathed cables at the Kennedy space center, prediction of the occurrence of geomagnetic storms, and esti- mation of the occurrence probability of giant freak waves in the sea area around Japan. Each of the above problems requires estimation of the extremal index γ in (1.2). Sev- eral estimators for γ have been proposed in the extreme value theory literature. One of the first estimators of γ is due to Pickands [11]. Peng [10] proposed a general Pickands type estimator. Another kind of extremal index is the moment estimator proposed by Dekkers et al. [6], which generalizes the Hill type estimator for positive γ (Hill [8]). However, there has been little work on trying to study the convergence properties of the estimators for γ. The question is: what is the penultimate form of the limit in (1.1)? Addressing this question is important because it will enable one to improve the modeling in each of the problems above. T he convergence properties of the Pickands estimator such as consistency, asymptotic normality and the strong convergence rate have been discussed by Dekkers and De Haan [5], De Haan [1]andPan[9]. Dekkers et al. [6] considered the weak consistency, strong consistency and the asymptotic normality of the Hill type estimator under different conditions. The aim of this paper is to consider the strong convergence rate of the Hill type estimator for γ under the second-order regularly varying conditions. The Hill type estimator for P(X i > 0) =1, i ≥ 1isdefinedby H n = 1 k(n) k(n)  i=1 logX n−i+1,n −logX n−k(n),n , (1.4) where X 1,n ≤ X 2,n ≤···≤X n,n are order statistics of X 1 ,X 2 , ,X n ,andk(n) are positive integers satisfying k(n) →∞, k(n)/n →0asn →∞.IfY 1 ,Y 2 , ,Y n are i.i.d random vari- ables with common distribution function Pr(Y 1 ≤ x) = 1 −1/x, x ≥1andifY 1,n ≤ Y 2,n ≤ ···≤Y n,n are the order statistics of Y 1 ,Y 2 , ,Y n then (U(Y 1 ),U(Y 2 ), ) d = (X 1 ,X 2 , ) and thus one may simplify H n as H n = 1 k(n) k(n)  i=1 logU  Y n−i+1,n  − logU  Y n−k(n),n  . (1.5) The investigation of the strong convergence rate of H n requires knowing the conver- gence rate of (1.3). For this, we need to define second-order regularly varying functions. Firstly, a measurable real function g(t)definedon(0, ∞) is said to be a general regu- larly varying function with auxiliary function a(t) if there exists a measurable function Z. Peng and S. Nadarajah 3 a(t) → 0(ast →∞) with constant sign near infinity such that lim t→∞ g(tx) −g(t) a(t) = S(x), (1.6) where S(x) is not zero for some x>0. It is known that S(x)mustbeoftheformc {x ρ − 1}/ρ (see, e.g., Resnick [12]) where ρ ∈is referred to as the index of regular variation. Now, suppose that there exists a regularly varying function A(t) → 0(ast →∞)such that lim t→∞ U(tx)/U(t) −x γ A(t) = H(x) (1.7) for all x>0, where H(x) is not a multiplier of x γ .Then,H(x)mustbeoftheformx γ {x ρ − 1}/ρ for some ρ ≤ 0, where ρ is the regularly varying index of A(t), and (1.7)islocally uniformly convergent (De Haan [1 ]). We say that U(t) satisfies second-order regularly varying conditions. It is easy to check that (1.7)isequivalentto lim t→∞ logU(tx) −logU(t) −γ logx A(t) = x ρ −1 ρ (1.8) for all x>0, which is also locally uniform convergent. 2. Main results We need the following four technical lemmas. Lemma 2.1. If k(n) satisfies k(n)/n → 0 and k(n)/(loglogn) ↑∞then lim n→∞ k(n) n Y n−k(n),n = 1 (2.1) almost surely. Proof. The result follows from Wellner [13] by noting that 1/Y i are uniformly distributed on (0,1).  Lemma 2.2. If k(n) ∼ α n ↑∞, loglogn = o(k(n)) and k(n)/n ∼β n ↓ 0 then limsup n→∞ ±  S n  k(n)  − μ n  k(n)  /  2k(n)loglogn = √ 2 (2.2) almost surely, where S n  k(n)  = k(n)  i=1 logY n−i+1,n , μ n  k(n)  = k(n)  logn −logk(n)+1  . (2.3) Proof. The result follows from Deheuvels and Mason [4] after noting that logY i are i.i.d standard exponential variables.  4 Strong convergence bounds of the Hill-type estimator Lemma 2.3. If k(n) ↑∞, k(n)/n ∼ β n ↓ 0 and loglogn =o(k(n)) then limsup n→∞ ± n/Y n−k(n)+1,n −k(n)  2k(n)loglogn = 1 (2.4) almost surely. Proof. TheresultfollowsfromDeheuvels[2, 3] after noting that 1/Y i are uniformly dis- tributed on (0,1).  Lemma 2.4. If (1.8)holdsthenforarbitrary > 0 there exists t 0 > 0 such that     logU(tx) −logU(t) −γ logx A(t) − x ρ −1 ρ     ≤  x ρ+ (2.5) for all x>1 and t>t 0 . Proof. follows from Drees [7].  Theorem 2.5. If (1.7)holdswithk(n) and A(n/k(n)) satisfying k(n)/n ∼ β n ↓ 0,  k(n)/(2loglogn) A(n/k(n)) →β ∈[0,∞) and k(n)/(logn) δ →∞for some δ>0 then limsup n→∞ ±  k(n) 2loglogn  H n −γ  ≤  √ 2+1  γ ± β 1 −ρ (2.6) almost surely. Proof. We prove the case for ρ<0. The proof for ρ = 0 is similar. One can write H n −γ = 1 k(n) k(n)  i=1 B i (n)A  Y n−k(n),n  + γ k(n) k(n)  i=1  logY n−i+1,n −logY n−k(n),n −1  + A  Y n−k(n),n  ρk(n) k(n)  i=1  Y n−i+1,n Y n−k(n),n  ρ −1  , (2.7) where B i (n) = logU  Y n−i+1,n  − logU  Y n−k(n),n  − γ log  Y n−i+1,n /Y n−k(n),n  A  Y n−k(n),n  −  Y n−i+1,n /Y n−k(n),n  ρ −1 ρ (2.8) for i = 1,2, ,k(n). By Lemmas 2.1 and 2.4,      k(n)  i=1 B i (n)      ≤  k(n)  i=1  Y n−i+1,n Y n−k(n),n  ρ+ (2.9) Z. Peng and S. Nadarajah 5 for all sufficiently large n.NotethatPr(Y ρ+ i ≤ x)=x −1/(ρ+) for 0 ≤ x ≤1andi=1, 2, , n. By [5, Lemma 2.3(i)] lim n→∞ 1 k(n) k(n)  i=1  Y n−i+1,n Y n−k(n),n  ρ+ = 1 1 −ρ −  (2.10) for almost surely. By Lemma 2.1 and since A(t) ∈ Rv(ρ), lim n→∞ A  Y n−k(n),n  A  n/k(n)  = 1 (2.11) almost surely. Hence limsup n→∞  k(n) 2loglogn   A  Y n−k(n),n    1 k(n)      k(n)  i=1 B i (n)      ≤  β 1 −ρ −  (2.12) almost surely. Letting  → 0, lim n→∞  k(n) 2loglogn A  Y n−k(n),n  k(n) k(n)  i=1 B i (n) = 0 (2.13) almost surely. Similarly, lim n→∞  k(n) 2loglogn A  Y n−k(n),n  k(n) k(n)  i=1  Y n−i+1,n Y n−k(n),n  ρ −1  = βρ 1 −ρ (2.14) almost surely. By Lemmas 2.2 and 2.3, limsup n→∞ ±  k(n) 2loglogn γ k(n) k(n)  i=1  logY n−i+1,n −logY n−k(n),n −1  ≤ limsup± γ  2k(n)loglogn  k(n)  i=1 logY n−i+1,n −μ n  k(n)   +limsup n→∞ ± γ  2k(n)loglogn  μ n  k(n)  − k(n) −k(n)logY n−k(n),n  ≤  √ 2+1  γ (2.15) almost surely. The result of the theorem follows by combining (2.13)–(2.15). Now, we provide an analogue of Theorem 2.5 when U(tx)/U(t)convergestox γ with faster speed. Specifically, suppose there exists a regularly varying function A(t) → 0 6 Strong convergence bounds of the Hill-type estimator (as t →∞)withindexρ ≤0suchthat lim t→∞ U(tx)/U(t) −x γ A(t) = 0, (2.16) where the convergence is locally uniform for x>0. It is easy to check that (2.16)isequiv- alent to lim t→∞ logU(tx) −logU(t) −γ logx A(t) = 0, (2.17) which is also locally uniformly convergent for all x>0. Under this assumption, the fol- lowing result holds. Its proof is similar to that of Theorem 2.5.  Theorem 2.6. If (2.16)holdswithk(n) and A(n/k(n)) satisfying k(n)/n ∼ β n ↓ 0 and  k(n)/(2loglogn) A(n/k(n)) →β ∈[0,∞) as n →∞then limsup n→∞ ±  k(n) 2loglogn  H n −γ  ≤  √ 2+1  γ (2.18) almost surely. Acknowledgment The authors would like to thank the Editor-in-Chief and the referee for carefully reading the paper and for their great help in improv ing the paper. References [1] L. De Haan, Extreme value statistics, Extreme Value Theory and Applications (J. Galambos, ed.), Kluwer Academic, Massachusetts, 1994, pp. 93–122. [2] P. Deheuvels, Strong laws for the kth order statistic when k  clog 2 n, Probability Theory and Related Fields 72 (1986), no. 1, 133–154. 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Zuoxiang Peng: Department of Mathematics, Southwest Normal University, Chongqing 400715, China E-mail address: pzx@swnu.edu.cn Saralees Nadarajah: Department of Statistics, University of Nebraska–Lincoln, Lincoln, NE 68583, USA E-mail address: snadaraj@unlserve.unl.edu . [6] considered the weak consistency, strong consistency and the asymptotic normality of the Hill type estimator under different conditions. The aim of this paper is to consider the strong convergence rate of. Japan. Each of the above problems requires estimation of the extremal index γ in (1.2). Sev- eral estimators for γ have been proposed in the extreme value theory literature. One of the first estimators of. STRONG CONVERGENCE BOUNDS OF THE HILL-TYPE ESTIMATOR UNDER SECOND-ORDER REGULARLY VARYING CONDITIONS ZUOXIANG PENG AND SARALEES NADARAJAH Received